Recently, a non-linear optical (NLO) wavelength conversion element that uses quasi-phase-matching in NLO crystal having a periodically poled structure has realized wavelength conversion with high efficiency. For example, a laser light source module that oscillates a visible laser beam can be obtained, in which the NLO wavelength conversion element converts the wavelength of ultra-red laser oscillated by a solid laser element to generate the second harmonic. Here, for the excitation light source of the solid laser element, a semiconductor laser element may be adopted.
The above NLO wavelength conversion element performs wavelength conversion with high efficiency when the phase-matching conditions are satisfied. For example, at the time of the second harmonic generation, high conversion efficiency can be achieved when the phase velocity of a non-linearly poled wave that is forcibly excited by the incident fundamental laser beam matches the phase velocity of the second harmonic generated by the non-linear polarization, bringing the light waves generated at the different positions of the element to the same phase so that coherent addition can be performed thereon. However, the NLO wavelength conversion element is temperature-dependent and changes its wavelength dispersion characteristics in accordance with the operating temperature. The operating temperature of the NLO wavelength conversion element therefore must be stabilized to prevent the phase-matching conditions from becoming unsatisfied.
The output light intensity and oscillation wavelength of the semiconductor laser element are also temperature-dependent. Under an environment of a temperature higher than the optimal operating temperature, the oscillation wavelength of the semiconductor laser element becomes longer. When a semiconductor laser is used as an excitation light source of a solid laser element that adopts a laser medium having a steep absorption spectrum such as yttrium vanadate (YVO4), a change in the oscillation wavelength of the semiconductor laser element is responsible for reduction of the output of the solid laser element.
Thus, the semiconductor laser element and the NLO wavelength conversion element should be maintained at certain temperatures to increase the optical output of a laser light source module incorporating a semiconductor laser element, a solid laser, and an NLO wavelength conversion element. The optical output of the laser light source module can be increased, of course, by exciting the solid laser element by use of a semiconductor laser array in which multiple semiconductor laser oscillators are arranged in a single element. With such a structure also, the entire semiconductor laser array should be maintained uniformly at a certain temperature, and the entire NLO wavelength conversion element should be maintained uniformly at a certain temperature from the aspect of increasing the optical output of the laser light source module.
For example, in a harmonic generator described in Patent Document 1, two Peltier devices are arranged on a substrate, with a semiconductor laser element on one Peltier device and an NLO wavelength conversion element fixed by a holding member onto the other Peltier device, so that the temperatures of the semiconductor laser element and the NLO wavelength conversion element can be controlled separately by these Peltier devices. The temperature of the semiconductor laser element is controlled in accordance with the temperature measured by a thermistor arranged in the semiconductor laser element, while the temperature of the NLO wavelength conversion element is controlled in accordance with a temperature measured by a thermistor arranged in the holding member to which the NLO wavelength conversion element is fixed.
Furthermore, Patent Document 2 describes a semiconductor laser device in which cooling means such as a Peltier device and a fin are arranged outside the casing of the module, and two heat transfer means are arranged inside the casing with a semiconductor laser element arranged on one heat transfer means by way of a heater, and a semiconductor electroabsorption optical modulator arranged on the other heat transfer means by way of a heater. In such a semiconductor laser device, the temperature of the semiconductor laser element is controlled in accordance with the temperature detected by a temperature sensor directly attached to the semiconductor laser element, while the temperature of the semiconductor electroabsorption optical modulator is controlled in accordance with the temperature detected by a temperature sensor directly attached to the electroabsorption optical modulator.
[Patent Document 1] Japanese Patent Application Laid-open No. H7-43759
[Patent Document 2] Japanese Patent Application Laid-open No. 2000-228556